Installing a SQL Server service pack silently with a script based approach is the aim of all my SQL Server installs. Since SQL Server 2008 the unattended install process has excelled and with added flexibility of scripting frameworks such as  Powershell , there are extra options for customising an install.

Maintaining SQL Server installation scripts assists in Disaster Recovery preparation

Once the script is created , it is easy deploy a service pack or cumulative update upgrade to thousands of servers in a efficient manner.

With a silent install , you have an option of applying the service pack to all instances on the server or just a specific instance

 The /quiet switch runs the update in unattended mode.  

--This completes a silent install of a service pack to a specific instance
E:\SQL_2008_R1_SP3\SQLServer2008SP3-KB2546951-x64-ENU.exe  /instancename=MYINST1 /quiet

--This completes a silent install of a service pack to a specific instance
E:\SQL_2008_R1_SP3\SQLServer2008SP3-KB2546951-x64-ENU.exe  /allinstances /quiet

 For a full list of switches check the msdn site

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